The present invention relates to a voltage regulating device for controlling bias voltage for developing of copying devices, etc.
The circuit shown in FIG. 2 is a bias circuit for developing of copying devices. This circuit permits a 20 kHz rectangular wave signal of a fixed duty cycle to be applied to the terminal 1 to make a transistor Q1 switch rapidly, meanwhile a 200 Hz rectangular wave signal to be applied to the terminal 2 to make a transistor Q2 switch. With the above construction, the voltage applied to the primary side of a transformer T1 via the rectifying and filtering circuits comprising a diode D1 and a capacitor C1 is varied to form voltage in the primary side of the transformer T1, and the induced voltage at the secondary side is rectified in the circuit composed of diodes D2 and D3 and capacitors C2 and C3 to form a doubled voltage, by which negative direct current voltage can be obtained at the terminal 3. The circuit is capable of varying the voltage level obtained at the output terminal 3 by adjusting a variable resistor VR1 for setting image density through varying the duty cycle of the rectangular wave signal input from the terminal 2.
However, in this circuit, characteristic variance of the transformer T1, etc may cause the variation of values of output voltage obtained at the terminal 3 as shown in FIG. 3 even when a rectangular wave of the same duty is input to the terminal 2. In FIG. 3, "a", "b", and "c", respectively, represent the maximum-voltage curve, the minimum-voltage curve, and medium voltage curve.
For the above reasons, the variable resistor VR1 is conventionally adjusted so that the output voltage is made same value when the signal applied to the terminal 2 is in a standard duty ratio. Therefore the above-mentioned adjustment is needed during the manufacturing processes, bringing an increase in production cost. Also values of output voltage obtained at the terminal 3, after adjustment, are varied (except Point A) as shown in FIG. 4. The variation is larger around the maximum and minimum output voltage levels, so that stable pictures cannot be obtained when the device is used for copy machines.